Many electronic equipment assemblies require electrical shielding between separate printed circuit boards. Electromagnetic waves (electromagnetic interference or "EMI") or radio frequencies (radio frequency interference or "RFI"), or, commonly known as electrical "noise," generated by one circuit board may adversely affect the operation of a second circuit board. However, a problem arises when two circuit boards that must be electrically shielded from each other must also electrically interface.
Numerous applications exist that require a means to electrically shield printed circuit boards, but at the same time provide a means for an electrical interface between the circuit boards. One particular application is a two-way radio used in police, fire and other emergency vehicles. The radio housing is divided into separate enclosures which mechanically isolate the individual circuit boards as a means to prevent electrical noise from one circuit board interfering with the operation of another circuit board. For example, the heat and/or noise generated by a power or transmitter circuit may interfere with the function of a receiver or logic circuit board.
However, the enclosures may not provide complete shielding between the circuit boards. This is because openings exist in the enclosure walls to allow multi-conductor cables to pass through to electrically interconnect the circuit boards. The openings provide a means for heat and noise to propagate from one enclosure to another and eliminate any isolation benefit of an enclosure.
A common way of interconnecting two isolated circuit boards without introducing EMI or RFI is to use feed-through capacitors. A feed-through capacitor mounts into an opening of the enclosure wall separating the circuit boards and is secured by soldering or mechanical means. A conductor passes through the center of the capacitor and the electrical signal is filtered by the capacitor. The noise currents are shunted to the enclosure through the dielectric material of the feed-through capacitor. This method may be convenient if only one or two conductors connect the circuit boards; however, if multiple conductors are required, this method is expensive, bulky, because multiple feed-through capacitors take up too much space, labor intensive and restrict the ease of assembling the electronic product.
Another method of connecting two isolated circuit boards is to use a multiple-conductor ribbon cable. The ribbon cable is easy to install and inexpensive. However, conventional ribbon cables do not have the capabilities of electrically shielding the conductors to filter out electrical noise. One of the disadvantages of using ribbon cable is that the installation process would require an extra step to shield the opening between the circuit boards in which the ribbon cable passes. The shielding procedure adds greater inefficiencies to the assembly of the electronic product that would outweigh the benefits offered by the ribbon cable.
Therefore, a means is needed to provide complete mechanical isolation and electrical shielding when interconnecting circuit boards in separate enclosures.